1. Field of the Invention
The present invention generally relates to improvements of a charge-coupled device (CCD) camera. More specifically, this invention relates to a device for preventing erroneous operation of the camera due to smear.
2. Description of the Prior Art
First, an example of a prior art smear reducer circuit will be described with reference to FIG. 1. Reference numeral 20 denotes generally a solid state image pickup device such as a charge-coupled device (CCD) and the like. If the solid state image pickup device 20 is of a frame transfer-type, it generally consists of a photosensor section 1, a storage section 2 and an output register section 3. On the surface of the photosensor section 1 is projected an image of an object (not shown) passed through a lens system (not shown). Thus, in the photosensor section 1 is formed an image of charge corresponding to an incident light. After one field period, namely, 1/60 sec elapses, the charge image in the photosensor section 1 is transferred to the storage section 2. The charge image transferred to this storage section 2 is further transferred to the output register section 3 in which it is converted to a serial signal and is then delivered as a pickup output.
Now, let us assume that, as shown in FIG. 1, on a part of the photosensor section 1 a relatively bright light part L is incident and in that part a charge image is formed. Then, when the charge is transferred from the photosensor section 1 to the storage section 2, the charges accumulated in the respective portions of the photosensor section 1 are passed through this incident light part L and electrons excited by this light appear. Accordingly, when this pickup output is reproduced by a cathode ray tube incorporated in a television receiver, as illustrated in FIG. 2, a bright image I corresponding to the incident light part L of FIG. 1 is reproduced and a bright band-shape image G with a width same as that of the image I is also reproduced in the vertical direction. The brightness of this image G is lower than that of the image I. The charge image corresponding to such band-shape image G on the solid state image pickup device 20 is generally referred to as smear.
Therefore, in the prior art smear reducer circuit of FIG. 1, the smear is avoided as follows. The pickup output from the output register section 3 is supplied to a movable contact a of a change-over switch (electronic switch is preferred) 5. One fixed contact b of the change-over switch 5 is connected to the input side of a one-line memory 6. The output side of the memory 6 is connected through an on-off switch 7 to the input side thereof. Reference numeral 8 denotes a differential amplifier which acts as a subtracter. The other fixed contact c of the change-over switch 5 is connected to the noninverting input terminal of the differential amplifier 8 and the output side of the memory 6 is connected to the inverting input terminal thereof. Numeral 9 denotes an output terminal led out from the differential amplifier 8.
The operation of the smear reducer circuit of FIG. 1 will be described with reference to the waveform diagram of FIG. 6. In FIG. 1, a hatched area represents a mask for shielding a light. Particularly in the photosensor section 1, a light shielding or mask portion 1a is formed on, for example, its upper end (lower end is also possible). A pickup signal S.sub.V1 (refer to FIG. 6A) derived from the solid state image pickup device 20 is supplied through the change-over switch 5 to the noninverting input terminal of the differential amplifier 8. This pickup signal S.sub.V1 contains a smear component m. When a signal from the mask portion 1a of the photosensor section 1 is delivered from the output register section 3, the movable contact a of the change-over switch 5 is changed in position to the fixed contact b to supply a smear signal Sm (refer to FIG. 6B) formed of only the smear component m to the memory 6. At this time, since the on-off switch 7 is turned on during substantially one field period, the smear signal Sm is stored in the memory 6 during substantially one field period. The movable contact a of the change-over switch 5 is selectively connected to the fixed contact c thereof in other time interval than one line period during which the aforesaid smear signal Sm from the mask portion 1a is delivered. Thus, in the differential amplifier 8 the smear signal Sm is subtracted from the pickup signal S.sub.V1 and then at the output terminal 9 a pickup signal S.sub.V2 (refer to FIG. 6C) with the smear component removed is obtained. In FIG. 6, reference letter 1H represents one horizontal period and HB a horizontal blanking period.
By the way, in the prior art smear reducer circuit of FIG. 1, the smear signal Sm derived from the one-line memory 6 contains a noise N so that, as shown in FIG. 6C, the noise N is superimposed upon the pickup signal S.sub.V2 finally obtained. Therefore, although the smear is reduced, a signal-to-noise (S/N) ratio of a reproduced picture screen is deteriorated.